To provide a regulated output current as a charger from a buck-type converter, typically a current sensing resistor is connected to the inductor to detect the current flowing therethrough by monitoring the dropout voltage on the current sensing resistor. But this current sensing resistor will reduce the efficiency and may cause thermal issue. For example, FIG. 1 is a diagram showing a popular structure in switching chargers by using a current sensing resistor, as disclosed in U.S. Pat. No. 6,369,561, LTC 4001 and SMB 135, in which the output current Io of a voltage regulator flows through a current sensing resistor Rsense so that the voltage drop on the current sensing resistor Rsense is proportional to the output current Io and, more specifically, the current flowing through the current sensing resistor Rsense and the output current Io have a fixed 1:1 proportional relationship therebetween. The voltage drop on the current sensing resistor Rsense is injected into the differential inputs of an error amplifier 10 to provide a signal to a controller 12 for feedback control, thus controlling a driver 14 to switch power transistors.
In the conventional method for output current detection of a voltage regulator, the current sensing resistor Rsense is located in the charging current path and therefore, it must be a precision resistor and have a low resistance to reduce the power loss on it. However, a low resistance can produce only a small voltage variation and a small voltage variation is sensitive to and easily interfered by noise.